1. Field of the Invention
The present invention relates to a multi-beam emitting device and a light scanning unit employing the same. More particularly, the present invention relates to a multi-beam emitting device which can decrease an interval between scanning lines and reduce crosstalk without approximating light emitting points, and a light scanning unit employing the same.
2. Description of the Related Art
A laser diode device (hereinafter, referred to as an LD device) can have a semiconductor structure in which, a plurality of light emitting portions provided on the same substrate radiate a multi-beam. A multi-beam scanning unit, which can simultaneously expose a plurality of scanning lines using the LD device, has found many practical uses.
Since the multi-beam scanning unit having a multi-beam light emitting device exposes a plurality of scanning lines at the same time, a driving speed of a light deflection unit, for example, the rotation speed of a polygon mirror, is reduced while maintaining the same or higher performance compared to a light scanning unit using a single beam. Thus, a high speed output is possible at a high resolution so as to guarantee low noise and high reliability. Accordingly, the multi-beam scanning unit is typically applied to image forming systems such as laser printers, copying machines, and facsimiles.
Japanese Patent Publication No. 2003-69152, the entire contents of which are incorporated herein by reference, discloses an example of the multi-beam light emitting device. According to the example disclosed in the Publication No. 2003-69152, an active region is provided on or above a sapphire substrate extending to a predetermined height and parallel to the sapphire substrate. Four active regions are arranged with a minimum pitch of 16 μm.
The conventional multi-beam emitting device and multi-beam light scanning unit however, typically have the following problems.
When the multi-beam emitting device is used for the image forming system, for example, when an image is output at a general pixel density (resolution) of 600 dpi, a distance D between scanning lines needs to be set to 42.33 μm. In this case, since the typically adopted magnifying power of a scanning optical system is 4-10 times, a sub-scanning light emitting point pitch d is required that is less than 10 μm.
Since the interval between the light emitting points is at least 16 μm in the example disclosed in the Japanese Patent Publication No. 2003-69152, the interval between the light emitting points needs to be decreased as described below to meet the above demand.
FIGS. 1 through 3 illustrate the arrangement of the conventional multi-beam emitting device. Referring to FIG. 1, an LD device 10 includes a substrate 11 and laser emitting portions 12 and 13 arranged on the substrate 11, such that a pitch interval of light emitting points is L1. Provided d equals a sub-scanning light emitting point pitch corresponding to a scanning line pitch, which corresponds to a resolution at a scanning surface, then a relationship in which d is less than L1 is satisfied. Thus, by inclining the substrate 11 by an angle θ1 with respect to a main scanning direction Dm, an exterior beam pitch is adjusted to match the sub-scanning light emitting point pitch d. The angle θ1 is obtained from an equation (1) in which,d=L1×tanθ1  (1)Accordingly, even when L1 is greater than d, L1 is the interval between the scanning lines corresponding to a resolution at the scanning surface.
The flux of light radiated from the laser emitting portions 12 and 13 is diffracted by active layers 12a and 13a such that the flux of light becomes an approximately oval beam having a major axis in a direction perpendicular to the active layers 12a and 13a. Accordingly, when the angle θ1 is small, a beam whose flux of light has a large diameter, is emitted from the LD device 10 in the sub-scanning direction.
In the light scanning optical system, a loss of light increases since the shape of a flux of input light needs to be corrected, typically by using an oval or rectangular aperture extending in the main scanning direction, to correctly arrange the position of the beam profile in the main and sub-scanning directions and the diameter of a spot.
The beam emitted from the LD device 10 is linearly polarized in a direction in which the active layers 12a and 13a extend. Thus, the beam emitted from the LD device 10 is incident as a P-polarized light on a light deflection unit, such as a polygon mirror. Since the P-polarized light is significantly dependent upon the angle of a reflection rate, problems can occur wherein a reflection spot occurrence, due to the scanning angle, increases. Thus, to compensate for the problem, a costly reflection film coating is needed.
As shown in FIG. 2, when an angle θ2, formed between the main scanning direction Dm and a plane surface of the substrate 11, is compared with the angle θ1 of FIG. 1, the loss of light is reduced as the LD device 10 is inclined to establish an inequity wherein θ2 is greater than θ1, but wherein the light emitting point pitch L2 on the substrate 11 approximately equals the sub-scanning light emitting point pitch d. In this case, as the two light emitting points approximately equal each other, thermal and electric cross-talk is generated such that light emission becomes unstable. In particular, in a state in which a first light emitting point emits light, when another light emitting point then starts to emit light, the light emitting output of the first light emitting point decreases.
As shown in FIG. 3, by setting the sub-scanning light emitting point pitch d at a value twice the scanning line interval D, that is, as shown in equation (2),d=2×D  (2)a beam spot 17 is interlace-scanned onto a photoreceptive drum 15. However, in this case, data process is complicated and thus an expensive image processing circuit is needed.
Accordingly, a need exists for a system and method to decrease an interval between scanning lines on a scanning surface while further reducing cross-talk between components.